This invention relates to a method for monitoring a continuously variable transmission (CVT) driven by an input unit, preferably for a motor vehicle. Continuously variable transmissions have a variator for adjusting the speed ratio between a lowest reduction step (LOW) and a highest reduction step (OD). The variators known work according to the toroidal or continuously variable principle.
One such continuously variable transmission usually is connected with an internal combustion engine or an electromotor by an input shaft. DE A 44 36 506 thus describes a continuously variable transmission which has a hydrodynamic converter with lock-up clutch, a reversing gear set, a first bevel pulley pair, a second bevel pulley pair, a drive belt and a hydraulic and electronic control device. The input shaft drives the hydrodynamic converter consisting of an impeller, a turbine wheel and a stator. With the hydrodynamic converter is associated a converter lock-up clutch. The turbine wheel and the converter lock-up clutch are connected with the transmission input shaft. The latter drives a reversing gear set by which the rotational speed of the transmission input shaft is directly transmitted to the first bevel pulley pair or the direction is changed for reverse gear. The variator is designed as belt drive variator having one bevel pulley pair, respectively, on an input and output shaft and a drive belt which rotates between the bevel pulley pairs, the bevel pulley pair situated on the input shaft having one bevel pulley stationary in axial direction and one primary pulley movable in axial direction by means of a first adjusting space and of a primary pressure-regulating valve and the bevel pulley pair situated on the output shaft having one bevel pulley stationary in axial direction and one secondary pulley movable in axial direction by means of a second adjusting space and of a secondary pressure-regulating valve independent of the primary pressure-regulating valve and having an electronic control unit which determines via electromagnetic actuators and hydraulic valves the pressure level of the adjusting space of primary pulley and secondary pulley. As a result of the axial mobility of primary pulley and secondary pulley, the rotation radius of the drive belt, here designed as sliding member belt, changes and therewith the ratio of the continuously variable transmission. The output takes place via an output shaft.
The continuously variable transmission, i.e. the CVT, is controlled by an electronic control unit by means of electromagnetic actuators and hydraulic valves, which regulate clutches and brakes and the pressure distribution thereof as a function of input variables such as a signal of a load position of the input unit, rotational speed of a transmission input shaft, rotational speed of an output shaft or temperature of a pressure medium.
The electronic control unit determines from input variables an operation point and adjusts the corresponding rotational speeds of the variator or the speed ratio of the CVT. If a serious error occurs, a transmission diagnosis function is activated and the CVT changes over to an emergency running operation. By emergency running operation is a state of the CVT in which there takes place no rotational speed regulation and no contact pressure regulation of the secondary pulley on the part of the electronic transmission control. To make possible this emergency running operation in which a constant pressure ratio or power ratio is maintained between primary pulley and secondary pulley with constant pressure level of the adjusting space of the secondary pulley, pre-controlled emergency valves are provided which act upon the pressure-regulating valves of the variator.
Important components of one such CVT are, in particular, the primary pressure circuit and the secondary pressure circuit for actuating the adjusting spaces of the variator formed by the bevel pulley pairs and the belt drive organ, on one hand, for the torque-dependent contact of the belt drive organ and, on the other, for setting the speed ratio. The primary pulley and the secondary pulley of the variator are controlled by pressure-regulating valves independent of each other which, in turn, are supplied by a superposed main pressure circuit.
Subject to principle, primary and secondary pressure-regulating valves are hydraulic components of high precision and essentially important with regard to the safety of the system. An undetected incorrect function of one of the valves can lead to an undesired speed ratio adjustment and thus to a driving state critical to safety.
The problem to be solved by the invention is to provide a method for monitoring a CVT with which incorrect operation of the primary pressure-regulating valve and/or of the secondary pressure-regulating valve is detected so that steps can be taken in both driving situations critical to safety and secondary failures in the transmission proper.